The present invention relates generally to recursive median filtering, and more particularly to filtering for digital imaging data.
Digital filters are used for signal processing in applications such as pulse code modulation (PCM) of voice signals, and digitally coded video signals. One field is FLIR (Forward Looking InfraRed) used on aircraft for navigation and targeting, particularly at night. FLIR imaging data typically has very small signal levels and a great need exists to improve signal to noise ratio.
Median filtering is a common image signal processing filtering technique. Its non-algebraic basis can provide a better filtering result under many conditions. Algebraic techniques such as linearly combining adjacent samples result in loss of high frequency signal content. Median filtering does not combine values but rather attempts to logically select the best data sample among the samples available and therefore more faithfully recreates the original signal.
United States patents of interest include No. 4,521,866 (Petit), which teaches a recursive digital filter comprising a recursive part in which there are circulating registers and a non-recursive part having serially connected registers. No. 4,539.690 (Speidel) also teaches a digital filter having a recursive part and a non recursive part. No. 4,583,234 (Ramadan) teaches a feedback circuit to provide enhanced amplitude bit information.